A substrate processing apparatus in which a wafer (a substrate to be processed) is taken out from a slot of a cassette having a plurality of slots each configured to accommodate a wafer, and subjected to a predetermined process such as a cleaning process, has been traditionally known. In the known substrate processing apparatus, the wafer is carried out from the cassette sequentially from upper side of slots and processed. Then, the wafer having been processed is carried in to the cassette sequentially from upper side of slots and disposed.
However, it is required to dispose the wafer at any slot in the cassette for several reasons such as, for example, to investigate the cause of troubles generated at the wafer, or to simplify the post-processing.
In this regard, the number of positions in which wafers are disposed in a substrate processing unit where a cleaning process is performed and a transfer unit that transfers the wafers in a substrate processing apparatus, is generally smaller than the number (e.g., twenty five sheets) of wafers to be accommodated in the cassette. Therefore, if a wafer having been processed was disposed at an arbitrary slot without any consideration, it is possible that the wafer having been processed may be placed on a slot in which an unprocessed wafer is not carried out therefrom yet, thereby overlapping with the unprocessed slot.
It has been known that while a wafer is being processed in a substrate processing unit, wafers received in a cassette other than the wafer being processed are re-arranged in a predetermined order based on wafer numbers. See, for example, Japanese Patent Application No. 2002-33369.